1. Field of the Invention
The present invention relates to linear actuators. More specifically, the present invention relates to a linear actuator with an internal dampening mechanism capable of adequately dampening the motion of the piston at the end of the stroke.
2. Description of the Related Art
Linear actuators are well known as an efficient and very flexible mechanism for producing constant and uniform linear motion. As a result, they are commonly used in many industrial settings that require or involve linearly moving an object or load. Hydraulic lifts, such as the type found on most heavy equipment or machinery, are but one well-known example of the way in which linear actuators have been used in industry.
A linear actuator comprises four main components: a housing, a piston (sometimes called a “piston rod”), a fluid, and a displacement mechanism (often called a “power supply”). In general, these components are arranged such that the displacement mechanism transfers or introduces power to the fluid. In turn, the fluid then transfers the power to the piston and causes the piston to undergo linear displacement. It is this transfer of power through the fluid that allows the actuator to achieve a uniform and constant mechanical motion of the piston.
In the simple actuator, the displacement mechanism used may be a pump, a motor, a mechanical lever, a pyrotechnic, or other mechanism that pressurizes the fluid. Generally, the displacement mechanism pressurizes the fluid so as to cause the fluid to exert a pressure or force upon the piston. This is preferably accomplished by either adding fluid from a fluid reservoir or by generating additional fluid through a chemical reaction or pyrotechnic event.
Typically, the fluid used in a linear actuator is a hydraulic fluid. Both liquid fluids and gaseous fluids may be used. As different fluids impart different characteristics to the actuator, the selection of the fluid, as well as the determination of whether the fluid should be a liquid or a gas, depends largely on the desired application.
The housing and piston are coupled together such that as the pressure is applied to the piston by the fluid, the piston extends or retracts linearly from the housing. The rate at which the fluid is pressurized usually determines the rate at which the piston moves. However, because the housing is generally secured to a stationary member or other solid surface, the position of the housing is usually fixed.
As the piston extends or retracts from the housing, the piston causes the moveable member to uniformly move in a desired direction. This moveable member or object is preferably positioned outside the actuator and is mechanically secured to the piston, although such features are not required. Usually, the piston causes the moveable member to undergo linear displacement. However, if the linear actuator is combined with gears, hinges, struts, or the like, rotary and/or non-linear motion may also be achieved.
Generally, the piston is a rod, ram, plunger, or other similar structure that is disposed within the housing. As a result, the terms “piston rod,” “ram,” or “plunger” may often be used interchangeably with the term “piston.” However, for the sake of simplicity and clarity, as used herein, the term “piston” refers to the member that extends or retracts from the housing, regardless of the specific shape, configuration, and/or orientation that is used.
In addition to the simple actuator described above, it is also common to encounter actuators that include one or more additional pistons. Such actuators are often referred to as “telescoping” actuators because the additional pistons slide within each other like in a telescope.
Linear actuators are further classified as either single-acting or double-acting. In a single-acting actuator, the fluid moves the piston within the housing in only one direction. An external force such as gravity returns the piston to a non-extended position. However, in a double-acting actuator, the fluid is directed such that the fluid may be used both to extend and retract the piston.
Pyrotechnic linear actuators are a particular type of single-acting actuator that have been used in the automotive safety industry. In a pyrotechnic linear actuator, the displacement mechanism is a pyrotechnic charge that may be ignited through an initiator. Ignition of the pyrotechnic produces a volume of hot gas, which in turn, exerts pressure and force upon the piston sufficient to cause the piston to extend from the housing. A particular advantage of pyrotechnic linear actuators is that they can be designed to produce rapid and reliable motion of the piston, and as such, have been employed as an instrument for deploying components designed to protect and aid a vehicle occupant during a crash or other accident.
More specifically, some pyrotechnic linear actuators have been used in association with modified vehicle bumper systems that are designed to increase the bumper's ability to protect the vehicle and/or the vehicle occupant during a crash. Usually such applications involve employing the actuator to deploy a reactive member, such as a deformable plate and/or all or a portion of a bumper, into a position that provides optimal impact protection to the vehicle.
Bumper airbag systems are another type of modified bumper system that have incorporated a linear actuator. Such systems are designed such that in the event of an accident, a linear actuator causes an airbag, similar to the type used on steering wheels and dashboards, to inflate on the exterior of the vehicle. Addition of a bumper airbag system has been shown to be desirable to reduce the severity of the accident by absorbing and dissipating a portion of the energy produced by the crash.
Unfortunately, many known linear actuators have significant limitations that affect their efficacy in both modified bumper systems and other mechanical systems generally. More specifically, many known linear actuators are heavy, bulky, and costly to produce, and thus, are unsuitable for many commercial applications in which space, weight, size, and/or cost are at a premium.
In addition, many known linear actuators, especially pyrotechnic actuators, are limited by the fact that they are “one shot” actuators. This means that the actuator may only be used once to deploy the piston. After this first use, the actuator must be discarded and replaced.
Perhaps more importantly, many known actuators fail to provide an adequate mechanism for dampening the movement of the piston at the end of the stroke. “Stroke” is the linear distance the piston travels from a fully retracted position to a fully extended position. In most situations, the displacement mechanism exerts sufficient pressure or force upon the piston to cause the piston to move at a high rate of speed. This is especially true in pyrotechnic actuators in which energy from the pyrotechnic event is used to extend the piston. Unless a mechanism or system is provided to dampen or brake the movement of the piston, the piston will reach the end of the housing at a high velocity, thereby causing the piston to forcibly impact the housing, the moveable member, or the external system employing the actuator. Such forcible impact is undesirable and may cause significant damage.
In the automotive safety industry, failure to adequately dampen the movement of the piston at the end of the stroke can be particularly detrimental. As noted above, the purpose of airbags and other safety components is to provide systems that reduce the severity of the crash by absorbing and dissipating a portion of the kinetic energy produced by the crash. However, if the movement of the piston is not adequately dampened, the piston will forcibly impact the vehicle and/or the airbag or other safety system. Such impact endangers the vehicle occupants because not only does it increase the kinetic energy and the severity of the crash, but also it may render the airbag or other vehicle safety components inoperable.
In an effort to combat these problems, some linear actuators have been developed with dampening mechanisms designed to dissipate the motion of the piston by compressing a fluid through an external orifice. Unfortunately, these mechanisms are limited by the fact that they add complexity, size, and bulk to the actuator. These dampening mechanisms also increase the costs associated with the production and assembly of the actuator. Furthermore, as such dampening mechanisms require extra components through which the fluid must flow, there is an increased likelihood that the actuator will leak and/or require maintenance to remain fully serviceable.
Accordingly, there is a need in the art for a novel linear actuator that addresses and/or solves one or more of the above-listed problems. Such a device is disclosed herein.